Tn7 transposition as a probe of cis interactions between widely separated (190 kilobases apart) DNA sites in the Escherichia coli chromosome.

We have used the bacterial transposon Tn7 to examine communication between widely separated DNA sites in the Escherichia coli chromosome. Using Tn7 target immunity, a regulatory feature of transposition which influences target selection, we have evaluated (i) how the presence of Tn7 sequences at one DNA site affects Tn7 insertion into another site in the same DNA molecule and (ii) the nucleotide distances over which the two sites are able to communicate. We demonstrate that Tn7 sequences at one chromosomal site act at a distance to inhibit insertion of Tn7 elsewhere in that DNA as far away as 190 kb, reflecting effective long-range cis interactions. We have found that while target immunity is effective over a substantial region of the chromosome, insertion of Tn7 into a more distant site 1.9 Mb away in the same DNA is not inhibited; this observation provides evidence that target immunity relies on DNA spacing. We also find that within the region of the chromosome affected by target immunity, the magnitude of the immune effect is greater at close DNA sites than DNA sites farther away, suggesting that target immunity is distance dependent. We also extend the characterization of the Tn7 end-sequences involved in transposition and target immunity and describe how Tn7 target immunity can be used as a tool for probing bacterial chromosome structure.